Guide

Make-up air units: filter changes and seasonal checks between cleanings

Updated 2026-10-07

The make-up air unit (MUA) is the rooftop box that pushes outside air back into the kitchen to replace what the hood pulls out. It’s usually nobody’s job. The hood cleaner works on the exhaust side, the HVAC contractor handles the dining-room units, and the MUA sits on the roof with clogged filters until the kitchen doors start slamming. Here’s what two major manufacturers put in their maintenance sections, laid out as a calendar.

Short answer Both manuals we checked say to look at the filters at least every 3 months, and more often in dirty conditions. CaptiveAire’s direct-fired heater manual adds a quarterly belt tension check. Yearly, it calls for inspecting bearings, belts, fasteners, and the motor, checking for gas leaks, cleaning the flame sensor, and inspecting the burner. Code-wise, the mechanical code requires make-up air whenever the hood runs, roughly equal to what it exhausts, so a dead MUA is a code problem, not just a comfort problem. Staff can watch the filters and notice the symptoms. Belts, bearings, and anything on the gas side belong to a qualified HVAC tech.

Maintenance calendar from manufacturer manuals

When	Task	Source	Who
2 weeks after start-up or a belt change	Re-check belt tension. New belts stretch.	CaptiveAire	HVAC tech
Every 3 months (sooner in dirty conditions)	Inspect filters; wash washable ones in warm soapy water or replace disposables. Reinstall with the airflow arrow pointing the right way.	Greenheck, CaptiveAire	Trained staff or facilities
Every 3 months	Check belt tension. Too tight wears bearings; too loose slips at start-up.	CaptiveAire	HVAC tech
Yearly	Inspect bearings and belts and replace worn ones; tighten bolts and set screws; clean dust and grease off the motor housing and the wheel.	CaptiveAire, Greenheck	HVAC tech
Yearly (gas-fired units)	Check for gas leaks, clean the flame sensor, inspect and clean the burner.	CaptiveAire direct-fired manual	Qualified gas or HVAC tech only
Motors with grease fittings	Re-grease per the motor maker; Greenheck’s general rule is about every 2,000 running hours when the motor stays under 104°F. Many small motors are sealed and never need it.	Greenheck	HVAC tech

That’s two manufacturers’ published schedules, not a universal rule. Your unit’s own manual wins, and the model and serial number are on the unit’s label if you need to find it.

Seasonal checks

Before heating season (gas-fired MUA): book the yearly burner inspection, gas-leak check, and flame-sensor cleaning before the first cold week, not after the unit locks out during a dinner rush.
After pollen, cottonwood, or construction dust: pull the filters early. The “every 3 months” is a minimum.
Long shutdowns (winter closure, seasonal kitchen): CaptiveAire’s steps are to switch off the external disconnect and close the inlet gas valve, then reverse both to restart. See low-volume kitchen minimums.
After hood cleaning day: confirm the MUA starts when the exhaust fan starts. The mechanical code expects them to run together.

Signs the MUA needs attention

Kitchen doors hard to open or slamming, or a whistle at the back door
Smoke rolling out past the front lip of the hood even though the exhaust fan is running
MUA not running when the hood is on, or a heater lockout light
Intake filters coming out greasy, which can mean the unit is pulling in your own exhaust. Have the HVAC contractor check intake and exhaust separation.
Squealing belt or a new vibration on the roof

Those are balance and equipment problems. Another hood cleaning won’t fix them. The step-by-step on who to call is in makeup air and exhaust balance.

Is the MUA in your hood cleaner’s scope?

Usually not, unless you write it in. Some public bid documents do. One school district’s hood cleaning bid priced each system to include the exhaust and make-up air fans. If you want one vendor on the roof for both, list the MUA filters and fan in the scope and ask for photos. See the RFP template and what a full-system clean includes.

Who it’s for / not for

For: kitchen managers and facilities staff who want a simple calendar for the supply side of the hood system, and operators writing an HVAC or hood vendor scope
Not for: burner adjustment, gas work, or airflow balancing. Those need a qualified technician. Not for sizing a new MUA either; that’s a design question for your mechanical engineer and building department.

Sources

CaptiveAire: Direct-fired heater installation, operation, and maintenance manual (PDF) (2-week, 3-month, and yearly maintenance; burner maintenance; prolonged shutdown; belt deflection 1/64 in. per inch of span)
Greenheck: Untempered make-up air units IOM (PDF) (filters at least every 3 months; yearly bearings, motor, wheel; 2,000-hour grease rule)
UpCodes: Commercial kitchen makeup air (IMC §508.1) (make-up air approximately equal to exhaust; mechanical make-up air runs with the exhaust)
ICC Building Safety Journal: Exhaust systems and makeup air under the 2024 IMC
Greenville County Schools: kitchen hood cleaning bid (PDF) (per-system price including exhaust and make-up air fans)

Next step

Find out when the MUA filters were last changed. If nobody knows, put a dated sticker on the unit the next time someone pulls them, and add a quarterly line for it to the same calendar as your hood cleaning.